Democrats have scrambled to contain the fallout for Hillary Clinton, their favoured 2016 presidential candidate, after allegations she inappropriately used her personal email for work while she was secretary of state.

But a spokesman for Mrs Clinton, Nick Merrill, said the politician had followed both the "letter and spirit of the rules" while she was secretary of state.

Mrs Clinton made no reference to the debate over her email use on Tuesday night, when she delivered a 30-minute speech at a gala dinner in Washington.

The State Department also defended Mrs Clinton.

"There was no prohibition on using a non-state.gov account for official business, as long as it's preserved," State Department spokeswoman Marie Harf said.

Democratic lawmakers and party loyalists cast Mrs Clinton's use of personal email as nothing unusual, noting that previous secretaries of state, including Colin Powell, used their personal accounts.

They also pointed out that when Republican George W Bush was president, senior adviser Karl Rove had used an address through the Republican National Committee to conduct some business.

A National Public Radio report said Chuck Hagel had not used an official account when he was defence secretary.

The rules governing high-level officials' emails have been in flux in recent years, so it is far from certain that any formal action will be taken against Mrs Clinton.

But it does provide ammunition for critics, especially for a congressional committee investigating the events surrounding the September 11, 2012, attack against a US diplomatic facility in Benghazi, Libya, in which the US ambassador to Libya was killed.

Trey Gowdy, the South Carolina Republican congressman who chairs the House Select Committee on Benghazi, said the committee learned recently Mrs Clinton had used more than one personal email address.

"The fact is the State Department cannot certify they have produced all of former secretary Clinton's emails because they do not have all of former secretary Clinton's emails, nor do they control access to them," he said.

Republican representative Jason Chaffetz of Utah said the House Oversight Committee, which he chairs, will work with the Benghazi committee "to further explore Hillary Clinton's use of personal emails while at the State Department".

Jeb Bush, a frontrunner for the Republican presidential nomination who recently released a trove of emails from his time as Florida governor, many sent on his personal account, demanded Mrs Clinton's emails be made public.

Records rules changed over Clinton's tenure

Officials acknowledge that Mrs Clinton used a personal email account throughout her State Department tenure.

The State Department said Mrs Clinton last year turned over emails from the period after a records request, and that 300 of these were sent to a committee investigating Benghazi.

A total of 55,000 pages of material covering the time she was in office were turned over, the agency said.

The Obama administration acknowledged the federal government was still in the process of modernising its policy on archiving emails and other digital information.

Federal agency heads were told in August 2012 they would have to start keeping electronic records of all emails.

A subsequent memo said explicitly that work performed through a personal email address may also need to be archived.

"The sole use of a private email account by a high-level official to transact government business is plainly inconsistent with the Federal Records Act and longstanding policies of the National Archives," lawyer Jason Baron said.

However, James Lewis, a technical expert at the Center for Strategic and International Studies, said the reality was many senior officials used personal email.

"The issue here was intent. Was she doing it deliberately to avoid having her emails tracked, and was there classified information?," he said.